The invention relates to a spray-coating apparatus with direct spray current measurement.
Spray-coating equipment is known from the European patent document 0160,179 B1. Through a keypad, an operator applies a desired voltage and a desired electric current that shall be kept constant by a regulation system. A threshold value applies to the distance between the spray equipment and the object to be coated. If the distance exceeds the threshold value, the high voltage at the high-voltage electrode of the spray equipment is kept constant independently of increasing object distance, the electric spray current of the high-voltage electrode decreasing as the object distance increases and increasing as it becomes smaller. When the distance is less than the said threshold value, the electric spray current is kept constant independently of changes in this distance because an increase in current being counteracted by a commensurate drop in voltage. Just before the object comes into contact with the spray equipment, the high voltage collapses entirely (contact protection). The electric current between the electrode and the object to be coated is determined indirectly by measuring the electric current in the primary of a transformer of which the secondary feeds a high-voltage cascaded circuit having rectifiers and capacitors generating the electrode""s high DC voltage DC.
U.S. Pat. No. 4,000,443 discloses spray equipment wherein a rise in the electric spray current when an object to be coated nears said equipment is counteracted by a commensurate reduction in the high voltage. The electric spray current is measured indirectly by the current in the secondary between the transformer and the high-voltage cascaded circuit. The European patent document 0,626,208 B1 discusses spray equipment wherein again the input voltage at a high-voltage generator is varied inversely to the current, the electric spray current being measured indirectly as a voltage drop across a resistor preceding the high-voltage electrode. It is known from the British patent 2,077,006 to counteract a rise in the electrode spray current by commensurately reducing the level of the high voltage within a predetermined operational window. U.S. Pat. No. 4,187,527 compares test values of electric spray currents measured in rapid succession when there is a sharp current rise indicating a short, in order to shut off the current.
The European patent document 0,559,608 B1 discloses measuring the current between the high-voltage electrode and the object to be coated using a meter mounted in the electrical path between said object and ground.
The objective of the invention is to solve the problem of preventing the dangers relating to excessively high voltages at the high-voltage electrode when there is a large distance from an electrically conducting object, or an excessively high current, or short, when the object is too near the object, where the object may be something to coat or an operator or something else, and at the same time to design the spray coating apparatus in such manner that optimal coating shall be achieved at minimal energy consumption.
This problem is solved by a direct measurement of the electric spray current in the return path from the object to be coated. As a result, true test results are provided and rapid response of the regulation system to deviations of instantaneous values from the reference values is achieved.
In especially advantageous manner of the invention, it contains a counter electrode at a potential substantially differing from that of the high-voltage electrode and for instance at ground in order that free electric particles, in particular that freely wandering ions and electrodes produced by the high-voltage electrode shall be shunted instead of accompanying the spray-coating material from the spray apparatus onto the object to be coated. Counter electrodes are known for instance from the European patent document 0,756,899 A2.